In the art, when a mobile terminal, such as a user equipment (UE), receives/transmits a voice call while attached to a Long Term Evolution (LTE) network in which voice service is not provided according to the underlying Radio Access Technology (RAT), the UE is commanded to fall back into an alternative RAT where the service is provided. Modern UEs, such as smart phones are generally capable of handling a plurality of different RATs, such as e.g. Bluetooth, Wi-Fi, 2G, 3G or 4G. Thus, there may be more than one alternative RAT where the service is provided, and typically one of them is the preferred alternative, e.g. a first system implementing Wideband Code Division Multiple Access (WCDMA) and a second system being Global System for Communication (GSM) where WCDMA is the first priority alternative RAT). A simplified coverage illustration of serving and alternative RATs is shown in FIG. 1. Thus, a base station 100 such as a base transceiver station (BTS) in case of GSM, a NodeB in case of Universal Mobile Telecommunications System (UMTS) or an eNodeB in case of LTE has certain coverage, where the serving RAT 110 is the current RAT servicing a UE 12 connected to the base station. From time to time, it may be necessary for a UE to change to an alternative RAT in case a particular service cannot be delivered by the serving RAT. Traditionally, this process has been done blindly at the base station 100, i.e. there is no information available about the signal quality of the alternative RATs 130, 140, and the changing from one RAT to another typically has taken tens of seconds. If the 1st priority alternative, or first target, RAT 130 has a good signal quality, the UE 120 will successfully attach and will be able to set up the call, as shown in FIG. 2.
Thus, in FIG. 2, the base station of the serving RAT determines whether a RAT change is to be undertaken on the basis of a service requested by the UE. Practically, the base station of the serving RAT may determine that a so called Circuit Switched Fall Back (CSFB) is to be undertaken, i.e. a method used for delivering voice services and/or Short Message Services (SMS) to LTE devices through the use of GSM or another circuit-switched network, since LTE—being a packet-based network—cannot support circuit-switched calls. The UE will hence fall back on a 3G or 2G network to complete the call or to deliver the SMS text message. The base station thus determines that a change of RAT must be made, i.e. the services requested by the UE (such as e.g. voice calls or SMS in LTE) cannot be provided for in the serving RAT, and sends to the UE an instruction to either change to a 1st priority alternative RAT or a 2nd priority alternative RAT. Thus, the UE tries to find a cell in the 1st priority alternative RAT, and if it is successful, i.e. if the quality is good enough, the UE will initiate an Attach process and subsequently set-up a call with the 1st priority alternative RAT.
With reference to FIG. 3, if the signal quality of the 1st priority RAT, i.e. the first target RAT, is not considered sufficiently good, the UE will still try to access that RAT for a certain period of time (specified by the UE manufacturer) until it decides to try and find a cell in the 2nd priority alternative RAT, i.e. the second target RAT, where it eventually may set up the call (if the signal quality is considered good enough). This will make the process of changing RATs long (up to 30 seconds in practical implementations), thus resulting in inferior accessibility and user perception of the network. This situation happens often in networks where LTE coverage is better than the 1st priority alternative RAT due to deployment frequencies, e.g. at sites where LTE is deployed in a 800 MHz band and UMTS is deployed in a 2100 MHz band.
With reference to FIG. 4, some network providers try to mitigate this by having the base station of the serving RAT commanding the UE to measure signal quality of the 1st priority alternative RAT prior to CSFB release, making sure that the alternative RAT only is measured if the signal quality is good enough. Thus, the base station commands the UE to measure the signal quality of the 1st priority alternative RAT prior to determining whether a RAT change is necessary, the measures the quality of the 1st priority alternative RAT and reports back to the base station of the serving RAT. When an CSFB is to be performed, the serving RAT is aware of the (high) quality of the 1st priority alternative RAT and sends an instruction to change to that RAT accordingly. The UE thus finds a cell in the 1st priority alternative RAT and initiates an Attach process and a call is subsequently setup with the 1st priority alternative RAT.
However, with reference to FIG. 5, should the quality of the 1st priority serving RAT not be sufficient, a report is accordingly sent from the UE to the base station, wherein the base station instructs the UE to find a cell in the 2nd priority alternative RAT. If the quality of the 2nd priority alternative RAT is good enough, a call is setup. Commanding the UE to measure signal quality of other RATs prior to CSFB adds at least 2-3 seconds to the call setup time and only works with UEs which support Packet Switched Handover (PSHO) to the RAT in question.